The present invention relates to rolling bearings partly or entirely made of a ceramic material used in a high temperature atmosphere.
The formation of an oxide film on the surface of a semiconductor wafer or the diffusion of impurities into the semiconductor wafer is performed in a high temperature furnace. The semiconductor wafer to be subjected to such treatment is usually placed on a carriage and transferred inside the high temperature furnace and after completion of the treatment, it is transferred outside the furnace together with the carriage.
Rolling bearings used for the wheels of the carriage are comprised of ceramic rolling members formed of balls or rollers, an inner ring and an outer ring both of which are made of steel. Also, the type are used that comprise an inner ring, an outer ring and rolling members all of which are made of ceramic rolling bearings.
That is, in the case of a rolling bearing having inner and outer rings and rolling members all made of steel material, it is necessary for the bearing to be lubricated with a lubricating oil so as to increase the antiabrasive as well as rotational efficiencies of the bearing but such lubricating oil tends to evaporate at high temperatures resulting in deterioration of these efficiencies. In contrast, a rolling bearing made of a ceramic material requires no lubrication.
The rolling bearing according to the present invention is of the type that includes at least a ceramic inner ring. However, where such a bearing is used for a carriage, the following problems arise.
That is, when the bearing having the ceramic inner ring attached to the steel shaft is used in a high temperature atmosphere, the shaft greatly expands with a temperature rise because there is a large difference in the coefficient of thermal expansion between the inner ring and the steel shaft (i.e., ceramics: 2.5-3.times.10.sup.-6, steel: 11.6.times.10.sup.-6) while the inner ring hardly expands as shown by the solid line A of FIG. 3 which is indicative of a distribution of the contact pressure between the inner ring and the shaft, an excessive stress concentrates on an edge 10a at each of the chamfered portions 10 formed at both axial ends of the inner ring 2. In this case, as the ceramic material is relatively weak as compared to steel, the edge portion 10a of the inner ring 2 is sometimes chipped or cracked. Further, due to the above disadvantage, the freedom of design relating to the fit between the shaft and the inner ring has been restricted so that it has been difficult to realize a structure satisfying the fit by which a favorable rotational efficiency is obtained.
Further, if, in the above case, the shaft is made of a ceramic material, the problem arising from the difference in the coefficient of thermal expansion between the shaft and the inner ring will be solved with surety but the cost will greatly increase. Therefore, it is quite unfavorable to form the shaft with the ceramic material especially for the above kind of carriage that is desired to be of a simple structure and to be manufactured at a low cost.
The present invention has been made in view of the above circumstances and an object of the invention is to provide a rolling bearing at least whose inner ring is made of a ceramic material and which is free of local concentration of stress on the inner ring due to the use of a steel shaft on which the inner ring is mounted and which is capable of bearing a heavy load without causing the ceramic inner ring to crack or chip.
Another object of the invention is to provide a roller bearing which allows of a wide range of freedom of designing a fitting structure between the shaft and the inner ring.